a. Field of Invention
This invention pertains to a device for exchanging data between two terminals simultaneously in both directions on one pair of wires.
b. Description of the Prior Art
Typically a relatively large computer installation or similar data processing center comprises one or more central computers which exchange data with a plurality of peripheral devices such as printers and so forth. Often these devices are several hundreds or thousands of feet away from the computer. Therefore it is common to use a local switching system in a star configuration for providing the means for said transmission. For relatively long transmission lines, duplex digital transmission of voice and data is fairly common. In such transmission various encoding schemes are used such as AMI and HDB-3. However because of actual length of these loops, various problems arise due to the long transmission times, signal attenuation, and reflection. Relatively expensive echo cancelling circuits are necessary to obtain acceptable signals. Furthermore these long loops preclude the use of a single power source for both the host and remote sets. Thus each set must be provided with its own power source.
In a full duplex mode two pairs of wires are commonly used to accommodate a digital signal exchange in two directions, each pair being dedicated to a direction. However providing four wires is rather expensive. Therefore two-wire systems have been proposed in which the wires are in effect dedicated consecutively for a first time interval to data communication in one direction, and for a second time interval they are dedicated to communication in the other direction. In this way messages are exchanged between the respective terminals in a ping-pong fashion. Of course in this configuration the overall data rate of the system can be maintained only by doubling the baud rate of the individual terminals.